Another family of reservoirs with photonic nodes is based on free-space optics principles. A configuration with a diffraction grating and Fourier imaging was designed with randomly interconnected microring resonators and applied to an imaging pattern task [1]}. Another reservoir with a diffractive optical element was recently described on the basis of an 8\(\times \) 8 laser array and a spatial light modulator. Rich interaction dynamics was experimentally observed, and the potential for scaling up to a complex configuration with low power consumption was illustrated [2]}, [3]}. Further modifications to this setup, including the use of a laser illumination field and digital micro-mirror device, led to the realization of an RNN with hundreds of nonlinear optical nodes applied to a time series prediction task [4]}. A reservoir in which node interaction comes from the scattering of light passing through a microscopic slide with a simple layer of white paint was also proposed and applied to logical function operation, with signal input via a micro-mirror array [5]}.
